Investigation of headed bar joints between precast concrete panels

Abstract

The paper addresses the design and behaviour of narrow cast in - situ joints between precast concrete elements in which continuity of reinforcement is achieved through overlapping headed bars. Using headed bars minimises the lap length required with in the cast - in - situ joint region. Confining reinforcement in the form of transverse bars and vertical shear studs is also installed in the joint. Th e paper describes a series of tensile tests which were carried out to simulate the tensile zone of a joint loaded in pure flexure . The headed bars used in the tests were 25 mm in diameter with 70 mm square heads and yield strength of 5 30 MPa. The tests studied the influences of concrete strength, headed bar spacing, splice length, transverse reinforcement and conf ining shear studs on joint strength . A lap length of 100 mm in concrete with 28 MPa cylinder strength was found to be sufficient to develop the full strength of the headed bars . A strut - and - tie model (STM) is presented for determining joint strength. Analys is shows that the STM gives safe results even though it does not fully capture the observed joint behaviour. An upper bound plasticity model is found to give relatively good predictions of joint strength in most cases , although it also does not always capt ure the correct failure mechanism . The tests provide insights into joint behaviour which, in conjunction with numerical modelling, will facilitate the development of an improved design method. W idespread use of this system would lead to improvements in bui ldability, sustainability and health and safety in the construction of concrete structures.